Independent Verification of the RPC TomoTherapy Audit Results

Purpose:MD Anderson Radiological Physics Center (RPC) routinely evaluates the calibration of linear accelerators using thermoluminescent dosimeters (TLDs) and more recently optically-stimulated luminescence dosimeters (OSLDs).The RPC continues to use TLDs for the annual TomoTherapy audits.In the past, an RPC measurement was 6.2% low compared to our institution planned dose.Therefore, we have constructed an RPC duplicate phantom and we have altered the design to accommodate an OSLD and an A1SL ionization chamber in order to examine the quality of the RPC TomoTherapy audit.

Methods:The RPC TomoTherapy Calibration Phantom consists of an acrylic cylinder with TLD capsules inserted in the center of the cylinder.A TomoTherapy treatment plan is generated to deliver a uniform dose of 600 cGy to an approximately 10 cm long by 2.5 cm diameter target that holds the TLDs.Two more treatment plans are generated for the in-house TomoTherapy Calibration Phantom; one for the OSLD and another for the A1SL ionization chamber. The beam energy and output calibration were checked prior to irradiating the phantoms. All Plans are delivered at the same time the RPC Phantom is irradiated for consistent comparison of the results.

Results:The results obtained with the in-house phantom were well within 2% of the calculated dose. The OSLD dose measurement was 1.8% from the dose calculated by the treatment planning system (TPS). The dose measured by the A1SL ionization chamber was only 1% from calculated. The repeated RPC measurement differed by only 2.8% from the calculated one.

Conclusion:We have manufactured an independent and similar dose verification phantom for testing the quality and reproducibility of the RPC TomoTherapy Calibration results. The phantom proved to be well suited for the evaluation of the overall machine calibration and for benchmarking the results of the RPC QA Audit. Institutions preparing for participation in the credentialing process will find this tool very helpful.